Paragonimiasis: A Parasitic Lung Disease
Paragonimiasis is a parasitic disease caused by several species of lung flukes belonging to the genus Paragonimus. These are flatworms that infect the lungs of humans and other mammals after they eat raw or undercooked crustaceans (such as crabs and crayfish) that contain the infective larvae of the parasite. Paragonimiasis can cause chronic cough, chest pain, bloody sputum, and sometimes fever and malaise. In rare cases, the parasite can migrate to other organs, such as the brain, and cause serious complications, such as seizures.
Pathology and Symptoms
Paragonimiasis is acquired when a person ingests metacercariae, the infective stage of the parasite, which are encysted in the tissues of freshwater crustaceans. The metacercariae excyst in the duodenum and penetrate the intestinal wall. They then migrate through the peritoneal cavity and diaphragm to reach the lungs. This migration can take several weeks and may cause abdominal pain and discomfort. In the lungs, the parasites mature into adult worms and form cysts (or granulomas) that communicate with the bronchi. The adult worms can live for several years and produce eggs that are either coughed up or swallowed and passed out in the feces.
The most common symptoms of paragonimiasis are chronic cough, chest pain, and bloody sputum (hemoptysis). These symptoms may mimic tuberculosis or lung cancer and often lead to misdiagnosis and delayed treatment. Some patients may also experience fever, night sweats, weight loss, and fatigue. The severity of symptoms depends on the number and location of parasites, the duration of infection, and the host immune response.
Paragonimiasis can also affect other organs besides the lungs. The most serious extrapulmonary manifestation is cerebral paragonimiasis, which occurs when the parasite migrates to the central nervous system. This can cause headache, dizziness, visual disturbances, mental changes, and seizures. Other possible sites of infection include the skin, subcutaneous tissues, abdominal cavity, liver, spleen, kidneys, and reproductive organs.
Epidemiology
Paragonimiasis is endemic in many parts of Asia, Africa, and Latin America. It is estimated that about 23 million people are infected worldwide, with more than 40 species of Paragonimus involved. The most common species that infect humans is Paragonimus westermani, also known as the oriental lung fluke. This species is prevalent in East Asia, especially China, Korea, Japan, Taiwan, Vietnam, Thailand, and the Philippines. Other important species include P. africanus in Africa, P. mexicanus in Central America, P. skrjabini in China and Russia, P. heterotremus in Southeast Asia, and P. kellicotti in North America.
The main risk factor for paragonimiasis is eating raw or undercooked freshwater crustaceans that harbor the metacercariae of Paragonimus. This practice is common in some cultures where crustaceans are considered delicacies or used for medicinal purposes. Some examples are raw crab marinated in vinegar (gejang) in Korea, drunken crab in China, raw crayfish in Louisiana, and crab juice used to treat asthma in Peru. In some cases, infection can also occur by eating raw or undercooked meat of animals (such as pigs and dogs) that have ingested infected crustaceans.
The transmission cycle of Paragonimus involves two intermediate hosts: a snail and a crustacean. The eggs of Paragonimus hatch in freshwater and release miracidia, which infect snails of various genera (such as Semisulcospira, Thiara, Bithynia, Melanoides). In the snail, the miracidia develop into cercariae, which emerge from the snail and infect crustaceans (such as crabs of genera Potamon, Eriocheir; crayfish of genus Cambaroides; shrimps of genera Caridina, Macrobrachium). In the crustacean, the cercariae encyst as metacercariae, which are infective to mammals.
Diagnosis
Microscopic examination of sputum, stool, or biopsy specimens for the presence of Paragonimus eggs. The eggs are oval, yellow-brown, and have a thick shell with a prominent operculum (lid) at one end. The eggs measure about 80-120 micrometers by 40-60 micrometers and are similar to those of some other trematodes (such as Clonorchis and Fasciola). Therefore, identification of the species may require molecular techniques or examination of the adult worms.
Serological tests that detect antibodies or antigens of Paragonimus in the blood or body fluids. These tests include enzyme-linked immunosorbent assay (ELISA), immunoblotting, immunofluorescence assay (IFA), and latex agglutination test (LAT). The serological tests are useful for screening and confirming the diagnosis, especially in cases of extrapulmonary paragonimiasis. However, they may have limitations such as cross-reactivity with other parasites, false-negative results in early or chronic infections, and false-positive results in previous or cured infections.
Imaging tests that show the characteristic lesions of paragonimiasis in the lungs or other organs. These tests include chest radiography, computed tomography (CT) scan, magnetic resonance imaging (MRI), and ultrasound. The imaging findings may vary depending on the stage and location of infection, but they typically show cystic or nodular lesions with calcification, fibrosis, cavitation, or pleural effusion in the lungs; ring-enhancing lesions with edema or hemorrhage in the brain; and abscesses or granulomas in the liver, spleen, kidneys, or reproductive organs.
Treatment
The treatment of paragonimiasis is based on anthelmintic drugs that kill or expel the adult worms from the body. The drugs of choice are praziquantel and triclabendazole, which are effective against most species of Paragonimus. Praziquantel is given orally at a dose of 25 mg/kg three times a day for two days. Triclabendazole is given orally at a single dose of 10 mg/kg. The treatment may be repeated after two weeks if necessary. The drugs are generally well tolerated and have few side effects, such as abdominal pain, nausea, vomiting, headache, dizziness, and rash.
The treatment may also require supportive care and management of complications, such as antibiotics for secondary bacterial infections, antitussives for cough, analgesics for pain, anticonvulsants for seizures, steroids for inflammation, and surgery for abscess drainage or cyst removal. The prognosis of paragonimiasis is good if treated early and adequately. However, chronic or untreated infections may result in permanent lung damage, neurological impairment, infertility, or death.
Prevention and Control
The prevention and control of paragonimiasis are based on interrupting the transmission cycle of Paragonimus and reducing the risk of human infection. The main strategies include:
- Educating the public and health workers about the disease and its transmission modes.
- Promoting safe food practices such as cooking crustaceans thoroughly before eating or avoiding eating them raw or undercooked.
- Improving sanitation and hygiene to prevent contamination of freshwater sources with human or animal feces.
- Controlling the snail and crustacean populations by using molluscicides or biological agents.
- Treating infected animals to prevent them from spreading the parasite to humans or other animals.
- Conducting surveillance and screening programs to detect and treat human cases early.
Paragonimiasis is a preventable and treatable disease that can cause serious health problems if left untreated. By following these measures, we can protect ourselves and our communities from this parasitic lung disease.
